1. Field of the Invention
This invention relates to methods of generating optical signals, and, in particular, to methods of efficient generation of microwave frequency (greater than 10 GHz) optical signals using semiconductor lasers for use in optical communication systems, signal processing, radar systems, and RF component testing. Accordingly, it is a general object of this invention to provide new and improved methods of such character.
2. General Background
It is desired to develop optical communications systems and optical modulation methods that will operate in mm wave (greater than 10 GHz) frequency range using semiconductor lasers. The advantages of using optical fibers rather than metal waveguides for distributing microwave signals is well known. For example, optical fibers offer the advantages of low cost, low attenuation, reduced size and weight, and freedom from electromagnetic interference.
While optical detectors with bandwidths .gtorsim.100 GHz have been fabricated, directly modulated semiconductor lasers or waveguide modulators are limited to frequencies .ltorsim.25 GHz. For example, high speed InGaAs detectors with bandwidths of 60 GHz have been reported by J. E. Bowers, C. A. Burrus and F. Mitschke, Electron. Lett. 22, 633 (1986). High speed GaAs detectors with bandwidths of 100 GHz have been reported by S. Y. Wang and D. M. Bloom, Electron. Lett. 19, pp. 554-555. The highest small signal bandwidth achieved at room temperature for a directly modulated semiconductor laser or external cavity traveling wave modulator is 22 GHz (R. Olshansky et al., Electronics Letters 23 pp. 839-841 (1987)) and 25 GHz, according to S. K. Korotky, G. Eisenstein, R. S. Tucker, J. J. Veselka, and G. Raybon, Paper FB4, Topical Meeting on Picosecond Electronics and Optoelectronics, Lake Tahoe, Nev. (1987). It appears that much improvement in semiconductor laser or integrated optic technology is required before devices can be developed with sufficient bandwidth and high power for modulation at the higher frequencies, such as at 35, 44, 60, and 90 GHz, which are of interest for many microwave and military applications (i.e., satellite communications, phased array radar, etc.).